A variety of engine/transmission mounts and mounting systems for controlling the vehicle engine and isolating the vibrations during vehicle operation have been developed. One such system is disclosed in U.S. Pat. No. 4,881,712 to Lun entitled Hydraulic Mount, issued Nov. 21, 1989 and assigned to the assignee of the present invention. The system disclosed in this patent includes a pair of engine mount assemblies having elastomeric bodies including two longitudinally aligned integral engine mounts. The mounts are in fluid communication with one another via an integral orifice track molded as a groove at the interface between the elastomeric body and a mating base plate. The engine mount assembly is mounted on a vehicle frame such that the common axis of the hydraulic chambers is situated transversely to the torque axis of the engine.
During operation, the engine is subjected to torsional movement or pitching about this torque axis. This torsional or pitching action of the engine causes one end mount of each assembly to compress forcing hydraulic fluid along the orifice track into the other mount. The other mount simultaneously expands and receives this excess fluid. Advantageously, the flow of fluid along the orifice track produces a desired damping effect of the vibration, including the torsional motion, to provide isolation from the vehicle frame.
The mounting system disclosed in the Lun patent is completed by the provision of a basic torque strut mounted between the top of the engine and the cross frame member to prevent excessive torsional motion. The torque strut comprises a single rigid piece vibrationally isolated from the engine and frame at its ends by rubber bushings. The torque strut and related engine mount assembly work in concert to provide acceptable control of the engine, as well as suitable isolation of the entire range of vibrations. However, in some vehicle/engine combinations, it is difficult to obtain the desired damping for certain vibrations, such as in the relatively high amplitude, low frequency range, especially when the torque strut is designed primarily for engine control. Also, some idling roughness can occur, due to the inability to decouple the strut.
Thus, while it can be said that the mounting system disclosed in the Lun patent is proven to be successful, it has been discovered that further improvements are needed. As indicated, a common diagnosis of problems in a vehicle is that resonant high amplitude, low frequency vibrations are present. In these cases, vibration isolation needs to be improved beyond what can be implemented by tuning of the main engine mount assemblies. Specifically, it has been found that for some engine/vehicle combinations, tuning of the torque strut to provide less damping, and thus a softer, overall torsional dynamic rate is needed for maximum performance. It is, of course, desirable to find a way this enhancement can be implemented without unduly sacrificing engine control or increasing the noise. Also, when resonant low amplitude, high frequency torsional vibrations are a problem, it is desirable to effect a decoupling of the torque strut.
Accordingly, a need exists for an improved torque strut and related mounting system for both controlling the engine and isolating vibrations, including particularly a full range of torsional displacements. Similarly, a need exists for a torque strut of improved design providing operating characteristics that may be tuned in order to achieve the most efficient and effective balance between control of engine motion and vibration isolation. Particular attention is also to be paid to torque decoupling during engine idle operation and increased damping capability of high amplitude, low frequency inputs. The torque strut and related mount assemblies should be matched to essentially eliminate any significant torsional vibration that would otherwise be transmitted through the vehicle frame to the passengers riding in the vehicle.